Hepatocytes (HC) derived by cell fusion of transplanted bone marrow cells have the capacity to proliferate and completely restore abnormal liver function in an animal model of metabolic liver disease, the fumarylacetoacetate hydrolase knockout mouse (Fah-/-). However, spontaneous fusion is rare and remains far below the threshold required for therapeutic use unless the fusion products have a strong selective advantage. Furthermore, basic properties of fusion-derived hepatocytes such as their ability to induce or participate in tumor formation are unknown. Preliminary data suggest that fusion-derived bone marrow transplants in the liver are often followed by high rates of tumor formation, possibly due to unknown mechanisms arising from cell fusion events. The overall goal of this application is to determine whether in vivo cell fusion could be a viable therapeutic strategy for the liver. To achieve this goal, I will attempt to clarify some of the basic properties of cell fusion in the liver, particularly focusing on characteristics associated with tumor induction. In addition, basic properties of cancer stem cells remain unknown in many tissues, including the liver. In an attempt to alleviate this, I will also determine the affects of the aging process on tumor stem cells in mice hepatocarcinomas. Aim 1: Determine if intrahepatic cell fusion events induce liver tumor formation. a. Examine the respective roles of cell fusion and NTBC withdrawal in liver tumor formation. Aim 2: Analyze the affects of the aging process on liver tumor stem cells. a. Transplant tumors from mice of varying ages and analyze resultant recipient tumors for differences. Relevance to Public Health: Cell fusion of bone marrow-derived cells has been shown to be a preferred mechanism for liver cell regeneration in vivo. More work is needed to determine the safety of this mechanism before it can be considered for human clinical use. Additional research on the characteristics and affects of aging on tumor stem cells and progenitor cells is required so that more specific treatments can be developed and made available for the fight against cancer. Corresponding and Relevant NIH 1C: National Cancer Institute